Regulation of HU alpha and HU beta by CRP and FIS in Escherichia coli.

The dimeric histone-like protein HU, one of the most abundant DNA binding proteins of Escherichia coli, is encoded by two closely related but unlinked genes, hupA and hupB. Overproduction of one or the other of the subunits has been shown to induce the SOS response and mucoidy. To understand how the synthesis of this protein is coordinated, we studied the transcription control of the two hup genes. We show here that CRP stimulated the transcription of both genes. In contrast, the FIS protein, one of the major positive regulators of the stable RNA operons, stimulated the transcription of the hupA gene, whereas it repressed that of the hupB gene. Moreover, stringent control, which like FIS also regulates the transcription of the stable RNA operons, affected the hupB transcription while it had no effect on hupA. This opposite regulation of the transcription of the two HU genes is reflected at the protein level signifying that changes in the composition of HU occur upon changes in the environment. It is rather unexpected that such divergent transcriptional regulation controls the two genes encoding a dimeric protein.